This invention relates to compensated CML logic circuits and more specifically to an improved integratable CML logic circuit which includes voltage and temperature compensation means.
A typical CML logic circuit arrangement may employ two transistors having their emitters connected to each other. The coupled emitters are then coupled through a constant current source transistor and an emitter resistor to a source of potential. The base of one of the two first-mentioned transistors and the base of the constant current source transistor and an emitter resistor to a source of potential. The base of one of the two first-mentioned transistors and the base of the constant current source transistor are normally coupled to one or more sources of reference potential, the base of the other of said two first-mentioned transistors serving as an input to the CML logic circuit.
The sources of reference potential may be derived from a voltage divider or taken directly from one or more voltage sources. The function of the constant current source transistor is to keep the overall emitter current of the two emitter-coupled transistors at a predetermined constant value. The value of the current is determined by the value of the emitter-resistor of the constant current source transistor, by the reference voltage applied to the base of the constant current source transistor, and by the emitter-base junction voltage of the constant current source transistor. Any variation in supply voltage or in the junction temperature will result in a variation of the voltage across the emitter-base junction of the constant current source transistor, since this voltage is strongly temperature dependent. If the voltage at the base of the constant current source transistor should be kept at a truly constant value with respect to ground by the reference potential, then variations in the voltage at the emitter of the constant current source transistor, as from fluctuations in junction temperature or from variations in power supply, would result in a varying current through the two commonly coupled transistors.
If the reference potential at the base of one of the two first-mentioned transistors is taken from the collector of a fourth transistor whose emitter is coupled through a resistor to a source of potential, and if the reference voltage at the base of the fourth transistor should be kept absolutely constant with respect to ground, the circuit will similarly fail to compensate for variations in supply voltage and junction temperature. Thus, variations in the input threshold levels and in variations in the levels of the output signals would result. Attempts have been made in the prior art to solve these temperature-related problems, as illustrated by U.S. Pat. Nos. 3,560,770 to J. Giles; 3,590,274 to R. Marley; and 3,622,799 to R. Marley et al.; but these prior attempts have not been totally successful. None of these attempts achieve the degree of total compensation possible with applicants' circuit. Only applicants' unique compensation compensates for fluctuations in junction temperature and for variations in power supply so as to insure that the output levels and input thresholds remain truly invarient.